Around the body, you want to right-click on your base layer, and choose 'blending options'. Then go to 'outer glow' and tick it on, then use the setting options to adjust it. Select the colour you want, by using the eyedropper tool and choose a colour coming from a light source on the image - if you don't want to do that choose a white that leans vaguely towards the main colour scheme of the image. This should do it for you :)
If you have more than one layer on your horse, try grouping them (the little folder icon at the bottom of the layers window), and right-clicking the folder.
Hope this helped!
Sunny
